1. Field of the Invention
The present invention relates to a battery pack in which a secondary battery, a control system for performing charge/discharge control of the secondary battery, and a transmission system for transmitting information on the secondary battery to an electronic apparatus using the secondary battery as a power source are packed into one unit. More particularly, the present invention relates to a remaining capacity calculation method for calculating the remaining capacity of the secondary battery from the data, a total capacity calculation method for calculating the present total capacity of the secondary battery, an overcharge protection method for protecting the secondary battery from excessive voltage (overvoltage) after the control system is reset, and an information display method for displaying information indicating the status and the like of the secondary battery.
2. Description of the Related Art
Conventionally, the main unit of a portable electronic apparatus, such as a video camera, uses a battery pack which is removably loaded as an operating power source, the battery pack being such that a secondary battery (hereinafter referred to simply as a battery), a microcomputer which controls charge/discharge of this battery, and the like are packaged into one unit. A microcomputer 21, such as that shown in FIG. 1, within a main unit 2 of such an electronic apparatus makes an inquiry to a microcomputer 11 of a battery pack 1 through a signal line 3 so that information (data), such as the remaining capacity of the secondary battery inside the battery pack 1, the number of charges/discharges (charge/discharge cycles), the current, the voltage, and various cautions (warnings) are transmitted. In response to this inquiry, when the microcomputer 11 of the battery pack 1 transmits the above-described information that has been acquired thus far by the microcomputer 11 through the signal line 3, the microcomputer 21 receives this information and performs various processing. Therefore, interactive communication is performed between the conventional battery pack 1 and the main unit 2 of the electronic apparatus.
The process performed by the microcomputer 11 of the battery pack 1 for the purpose of acquiring the above-described information includes a process for calculating the capacity remaining (remaining capacity) in the secondary battery. In order to calculate this remaining capacity the microcomputer 11, having a reference table of one discharge curve with no load, calculates the capacity during no load by adjusting for current and temperature on the voltage data during an actual load by referring to this reference table, and transmits this capacity as Wh (or Ah) information to the main unit 2. The main unit 2 adjusts for current and temperature on the received capacity during no load and determines the remaining capacity as time information.
When the microcomputer 11 of the battery pack 1 determines the total capacity after a charge/discharge cycle, conventionally, a .DELTA.V method has been adopted. In this method, current is cut off once during charging, a voltage drop .DELTA.V at that time is read to compute the impedance of the battery, and the total capacity is computed on the basis of the ratio of this result to the initial impedance value of the battery.
When the voltage of the battery falls below a voltage at which the microcomputer 11 can operate normally, the microcomputer 11 of the battery pack 1 enters a state in which the terminal voltage of the battery may not be determined accurately. In the conventional case, even if the microcomputer 11 enters such a state, detection of overvoltage of the battery is performed nonetheless, and overcharge control of the battery is performed.
It is common practice that the main unit of the electronic apparatus checks information indicating the status of the battery, such as the capacity of a conventional battery pack after the battery pack is loaded into the main unit of the electronic apparatus, and then displays the information on a display device or the like connected to the electronic apparatus.
In the interactive communications performed between the conventional battery pack 1 and the main unit 2 of the electronic apparatus and the like into which it is loaded as described above, the battery pack 1 constantly acquires up-to-date information and waits in preparation for an inquiry from the main unit 2. In order to meet such a demand, the microcomputer 11 of the battery pack 1 must perform high-speed processing (the frequency of the operating clock is high). Therefore, there are problems in that an expensive microcomputer must be used and the consumption of power is increased. Also, the main unit can only obtain the information of the battery pack 1 when the main unit makes an inquiry, and a load is correspondingly imposed on the microcomputer 21 on the main unit side.
Furthermore, since two adjustment calculations are performed in the step of determining the remaining capacity of the battery pack 1, calculations become complex, and since the error of adjustment calculations due to, in particular, temperature is large, the calculation accuracy is deteriorated. Furthermore, since this remaining capacity is computed by assuming that the present load conditions continue thereafter, in the worst case of maximum load being applied, the remaining capacity cannot be known, the battery runs out earlier than the computed remaining capacity, and the user becomes perplexed.
In addition, in the computation of the total capacity by the conventional .DELTA.V method, high-accuracy prediction adjustment is required, in particular, for the relationship between temperature versus internal resistance, and the adjustment is complex. Therefore, the accuracy of the computed total capacity is decreased. Since a prediction adjustment is also performed in the calculation for determining the total capacity after a charge/discharge cycle, calculations become complex, and the calculation accuracy is deteriorated.
When the voltage of the battery falls below the reference voltage at which the microcomputer 11 can operate, the overvoltage detection operation for the battery thereafter becomes unstable, and the possibility remains that the battery may be overcharged.
In order to display information indicating the status of the battery of the battery pack, the user must load the battery pack into the main unit, this becomes a burden to the user, and the status of the battery pack cannot be checked in advance.